The long range objective of the proposed research is the study of the socioeconomic and biological determinants of fertility. During the first year of this research (the actual period covered by this proposal) the construction of a model is planned which relates a set of intermediate fertility variables (e.g., age at first marriage, fecundability, fetal wastage, duration of post-partum anovulation) to fertility measured by age-parity specific birth rates. The methodology used for the construction of this model is based on Systems Theory. The model is built in three phases: 1) a birth-interval model relates a subset of intermediate fertility variables to the distribution of intervals between successive births in a hemogeneous group of women. 2) a family building model combines birth interval distributions to give the distribution of births to a homogeneous marriage cohort. 3) the fertility model combines the distributions of births to different marriace cohorts, resulting in an expression for age-parity specific birth rates. The inputs for this model will be estimated from and the computer simulation outputs compared with a set of empirical data from Bangladesh. This model can be used to analyze and interpret any set of fertility data from natural fertility populations or from populations using IUD's or induced abortion as means for birth control. The model may serve to refine population projections and can be applied for the planning and evaluations of birth control programs.